Extract *.sc3 file(s) into the Cities directory of the SimCity 3000 folder. It is likely that the directory is C:\Program Files\Maxis\SimCity 3000\Cities.
Double-click on the *.sc3 file to open it! You can also open a city(*.sc3) using the Load City option in SC3K!
Extract *.bld file(s) into the buildings directory of the SimCity 3000 Building Architect Tool folder. It is likely that the directory is C:\Program Files\Maxis\SimCity 3000 Building Architect\Buildings.
Double-click on the *.bld file to open it! You can view and/or edit the building(*.bld) in the SC3K Building Architect Tool using the Load Building option! You can also include the building in a Building Set so that you can use it in your own game!